According to the principles of the instant invention, there are disclosed photovoltaic devices and method for the fabrication of photovoltaic devices which exhibit a substantially uniform absorption of photons from the solar spectrum (typically Global AM 1.5 illumination), and consequently a uniform distribution of charge carriers throughout at least a substantial portion of the bulk of the photoactive region thereof. As a result, the peak rate of charge carrier (defined as electron-hole pair) recombination is reduced and the photovoltaic devices are rendered less sensitive to the effects of light induced defects formed therein. Long term photoconversion stability is thereby improved.
Recently, considerable efforts have been made to develop systems for depositing amorphous semiconductor materials, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n type photovoltaic devices which are, in operation, substantially equivalent to their crystalline counterparts. It is to be noted that the term "amorphous", as used herein, includes all materials or alloys which have no long range order, although they may have short or intermediate range order or even contain, at times, crystalline inclusions.
It is now possible to prepare amorphous silicon alloys by glow discharge or vacuum deposition techniques, said alloys possessing (1) acceptable concentrations of localized defect states in the energy gaps thereof, and (2) high quality electrical and optical properties. Such deposition techniques are fully described in U.S. Pat. No. 4,226,898, entitled Amorphous Semiconductors Equivalent To Crystalline Semiconductors, issued to Stanford R. Ovshinsky and Arun Madan on Oct. 7, 1980; U.S. Pat. No. 4,217,374, of Stanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980, also entitled Amorphous Semiconductors Equivalent To Crystalline Semiconductors; and U.S. Pat. No. 4,517,223 of Stanford R. Ovshinsky, David D. Allred, Lee Walter, and Stephen J. Hudgens entitled Method Of Making Amorphous Semiconductor Alloys And Devices Using Microwave Energy, which issued on May 14, 1985. As disclosed in these patents, which are assigned to the assignee of the instant invention and the disclosures of which are incorporated by reference, fluorine introduced into the amorphous silicon semiconductor layers operates to substantially reduce the density of the localized defect states therein and facilitates the addition of other alloying materials, such as germanium.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was described at least as early as 1955 by E. D. Jackson in U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein disclosed utilized p-n junction crystalline semiconductor devices. Essentially, the concept employed different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc). The tandem cell device (by definition) has two or more cells with the light directed serially through each cell. In the first cell a large band gap material absorbs only the short wavelength light, while in subsequent cells smaller band gap materials absorb the longer wavelengths of light which pass through the first cell. By substantially matching the generated currents from each cell, the overall open circuit voltage is the sum of the open circuit voltage of each cell, while the short circuit current thereof remains substantially constant. However, it is virtually impossible to match crystalline lattice constants as is required in the multiple cell structures of the prior art. Therefore, tandem cell structures cannot be practically fabricated from crystalline materials in a manner which would have commercial production ramifications. As the assignee of the instant invention has shown, however, such tandem cell structures are not only possible, but can be economically fabricated in large areas by employing amorphous materials.
The multiple cells preferably include a back reflector for increasing the percentage of incident light reflected from the substrate back through the semiconductor layers of the cells. It should be obvious that the use of a back reflector, by increasing the use of light entering the cell, increases the operational efficiency of the multiple cells. Accordingly, it is important that any photoresponsive layer of semiconductor material deposited atop the light incident surface of the substrate be transparent so as to pass a high percentage of incident light from the reflective surface of the back reflector through the layers of semiconductor material.
Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, amorphous silicon alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Such continuous processing systems are disclosed in the following U.S. Pat. No. 4,400,409, for A Method Of Making P-Doped Silicon Films And Devices Made Therefrom; U.S. Pat. No. 4,410,588, for Continuous Amorphous Solar Cell Production System; U.S. Pat. No. 4,438,723, for Multiple Chamber Deposition And Isolation System And Method; U.S. Pat. No. 4,492,181 for Method and Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells; and U.S. Pat. No. 4,485,125 for Method and Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells; and pending U.S. patent application Ser. No. 244,386 filed Mar. 16, 1981 for continuous Systems For Depositing Amorphous Semiconductor Material. As disclosed in these patents and application, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific semiconductor material. In making a photovoltaic device of p-i-n type configurations, the first chamber is dedicated for depositing a p-type semiconductor alloy, the second chamber is dedicated for depositing an intrinsic amorphous semiconductor alloy, and the third chamber is dedicated for depositing an n-type semiconductor alloy. Since each deposited semiconductor alloy, and especially the intrinsic semiconductor alloy, must be of high purity; every possible precaution is taken to insure that the sanctity of the vacuum envelope formed by the various chambers of the deposition apparatus remains uncontaminated by impurities, regardless of origin.
The layers of semiconductor alloy material thus deposited in the vacuum envelope of the deposition apparatus may be utilized to form photoresponsive devices, such as, but not limited to photovoltaic cells which include one or more p-i-n cells or one or more n-i-p cells, Schottky barriers, photodiodes, phototransistors, or the like. Additionally, by making multiple passes through the succession of deposition chambers, or by providing an additional array of deposition chambers, multiple stacked cells of various configurations may be obtained.
As should be apparent by the foregoing, thin film amorphous semiconductor materials offer several distinct advantage over crystalline materials, insofar as they can be easily and economically fabricated into large area photoresponsive devices by newly developed mass production processes. However, heretofore produced amorphous silicon based semiconductor materials were prone to degrade as a result of prolonged exposure to light. This process, termed "photodegradation", or "Staebler-Wronski degradation," although not fully understood, is believed to be due to the fact that long-term exposure to a photon flux tends to break the bonds between the constituent atoms of the semiconductor material, thereby resulting in the formation of defect states in the band gap, such as dangling bonds, which are detrimental to the photovoltaic efficiency of a photoresponsive device which incorporates the degraded semiconductor material. It has further been observed that photogenerated defects may be annealed out of a sample of degraded semiconductor material by exposing said sample to elevated temperatures; for example, temperatures of approximately 150.degree. degrees for several hours. Samples of semiconductor material, thus degraded by operational exposure to light and subsequently annealed, are restored to approximately the same level of photovoltaic performance which they exhibited prior to said operational degradation.
It is somewhat parodoxical that the higher the initial (pre-operational) quality of the photovoltaic semiconductor material, (1) the greater the effect of photodegradation thereupon, and (2) the greater the operation-dependent loss of efficiency exhibited by a photovoltaic device incorporating such higher quality semiconductor materials. The reason for this phenomenon is that lower quality photovoltaic semiconductor material initially includes a relatively high number of defect states therein and consequently, the formation of additional defect states in the energy gap thereof via photodegradation is not as significant as for a higher quality semiconductor material which is initially characterized by a relatively low number of defect states. Because of the fact that the assignee of the instant invention is now able to commercially manufacture high quality photovoltaic semiconductor materials exhibiting a low initial density of defect states, in a high volume, continuous production process, the problem of photodegradation of photovoltaic devices fabricated therefrom has become increasingly significant. The practical ramifications of the foregoing is that the consumer is not interested in expending large sums of money on photovoltaic energy generating systems which will lose upwards of 20% efficiency over their operating life.
Heretofore, the effects of photodegradation were dealt with by either (1) annealing the semiconductor material to remove the defect states and restore its electrical generating capacity, or (2) ignoring the defect states and allowing the semiconductor material to operate at less than full efficiency. Neither of the aforementioned options is commercially acceptable. While annealing does restore photodegraded cells to their initial operating efficiency, it necessitates the inclusion of additional hardware in a photovoltaic power system at additional cost to the consumer, and might also entail the periodic expenditure of labor. Several methods of annealing have been proposed. In one such method the annealing procedure may be instituted on a cyclic basis wherein the semiconductor material is periodically, typically at an interval of months to years, heated to an elevated temperature for a period of time sufficient to remove the defect states therein and restore the initial efficiency thereof. The heating may be carried out in situ by including a heat source in the photovoltaic installation, or the semiconductor material may be removed from the point of use and heated in an oven. In an alternative process, the semiconductor material may be continuously annealed by incorporating said material into a solar collector panel, which panel is adapted to collect and retain the solar thermal energy incident thereupon. In such an arrangement the semiconductor material is maintained, during normal operation, at the elevated annealing temperature and the formation of defect states as well as the annealing of those defect states, occurs simultaneously. Depending upon the operating temperature of the semiconductor material, overall degradation can be prevented or substantially slowed down. Such methods and techniques of continuous annealing are disclosed in U.S. patent application Ser. No. 636,172 of Vincent D. Cannella entitled, "Photovoltaic Panel Having Enhanced Conversion Efficiency Stability", filed July 31, 1984 and assigned to the assignee of the instant invention, the disclosure of which application is incorporated herein by reference.
As mentioned supra, in the second alternative, the amorphous photovoltaic devices which incorporate the semiconductor material are simply allowed to photodegrade. The rate of photodegradation for a particular device configuration may be readily ascertained, and the power requirement for a given installation may be therefore readily specified to account for the degree of photodegradation expected during the operational life of the photovoltaic devices. For example, it may be predicted that a particular photovoltaic device will degrade to 80% of its initial electrical performance within a period of 10 years of operation; therefore a built-in excess capacity of 20% may be incorporated in the initial installation to account for this subsequent loss. While such an approach is relatively simple and may be acceptable for a variety of photovoltaic installations, it is obviously a less than adequate solution to the problem, and represents an intolerable solution for many other uses. In installations in which space for solar collection is at a premium, it is clearly desirable to have the photovoltaic devices operate at their maximum capacity at all times. In other installations reliability and consistency of electrical power generated by and delivered from the devices is required. In such installations, the photovoltaic devices must be fabricated from semiconductor material which is relatively consistent in its conversion efficiency throughout the expected operational lifetime thereof.
As should be appreciated from the foregoing discussion, it would be highly advantageous and commerically necessary to provide a thin film amorphous photovoltaic device fabricated from semiconductor material which does not require the inclusion of extraneous hardware therewith, but which is nonetheless capable of maintaining a consistently high conversion efficiency under long term, high photon flux operating conditions, i.e., does not markedly degrade when exposed to light.
As previously stated, the mechanism of photodegradation of amorphous photovoltaic semiconductor materials is not fully understood; however, it is believed that said photodegradation involves the production of a wide distribution of defect states in the band gap of the semiconductor material. The term "defects", or "defect states" as generally used by routineers in the field of amorphous semiconductor materials, is a broad term generally including all deviant atomic configurations such as: broken bonds, dangling bonds, bent bonds, strained bonds, vacancies, microvoids, etc. In a photovoltaic device, a charge carrier pair (i.e. an electron and a hole) is generated in response to the absorption of photons from incident radiation in the photoactive region of the semiconductor material thereof. Under the influence of an internal electrical field established by the doped layers of semiconductor material of the photovoltaic device, such as a solar cell, the charge carriers are drawn toward opposite electrodes of the cell causing the positively charged holes to collect at the positive electrode and the negatively charged electrons to collect at the negative electrode thereof. Under ideal operating conditions, every photogenerated charge carrier will be conducted to its respective collection electrode. However, operating conditions are not ideal and the loss of charge carriers occurs to some degree in all photovoltaic cells. The primary charge carrier collection loss is due to charge carrier recombination, wherein an electron and a hole reunite. Obviously, charge carriers that reunite or recombine are not available for electrode collection and the resultant production of electrical current. Defects or defect states that occur in the photoactive region of the semiconductor material of the photovoltaic device provide recombination centers which facilitate the reunion and recombination of electrons and holes. Therefore, the more defects or defect states that are present in the semiconductor material of a device, the higher the rate of charge carrier recombination therein. Accordingly, charge carrier collection efficiency decreases as the rate of charge carrier recombination increases within the photoactive region of a given semiconductor material; an increase in the number of defect states is therefore, at least partially responsible for an increase in the rate of charge carrier recombination and a concomanitant decrease in photovoltaic cell conversion efficiency.
In photovoltaic cells of the type which comprise a layer of intrinsic amorphous semiconductor material having a layer of p-type semiconductor material disposed on one side thereof and a layer of n-type semiconductor material disposed on the other side thereof, referred to hereinafter as p-i-n type photovoltaic cells, applicants have observed a dramatic decrease in blue response (conversion of photons from the blue portion of the solar spectrum into electrical current) in the photoactive regions of the semiconductor material thereof relative to the red response (conversion of photons from the red portion of the solar spectrum into electrical current) in the photoactive regions of the semiconductor material thereof, upon photodegradation. That is to say, when such photovoltaic cells are exposed to a high intensity photon flux, the photoconversion efficiency measured under blue illumination decreases much more than the photoconversion efficiency measured under red illumination. The terms "blue illumination" or "blue light" are defined herein as having a wavelength within the approximate range of 350 to 550 nanometers; and the terms "red illumination" or "red light" are defined herein as photons having a wavelength within the approximate range of 550 to 750 nanometers. It is known that the absorption coefficient (the rate at which photons absorbed as they generate electron-hole pairs) of blue light in amorphous silicon alloy materials is greater than the absorption coefficient of red light in amorphous silicon alloy materials; therefore, under illumination equivalent to standard terrestrial conditions blue light is almost totally absorbed in the first thousand angstroms of the photoactive region of the semiconductor material of the photovoltaic cell, whereas the absorption of red light occurs more uniformly throughout the bulk of said photoactive region. Therefore, under blue illumination a very high density of charge carriers is generated in the first thousand angstroms of the photoactive region of the cell. In an undegraded photovoltaic cell, few defect states are present to provide recombination sites, and consequently the charge carriers are efficiently collected by the respective electrodes of the cell despite the high density thereof. However, if the photovoltaic cell is photodegraded, the high density of defect states therepresent provides recombination centers which facilitate the recombination of electrons and holes. Furthermore, the high density of electrons and holes created under blue illumination is conducive to, and greatly facilitates said recombination, since statistically, an electron and a hole are more likely to be reunited at a recombination center under high density conditions. Therefore, the collection efficiency of charge carriers, and the resultant overall cell performance under blue illumination is correspondingly decreased.
Photons of red illumination passing through the semiconductor material of a photovoltaic cell are not as readily absorbed as are photons of blue illumination. Consequently, said red photons penetrate a further distance through the bulk of the photoactive region of the semiconductor material of the photovoltaic cell. Charge carriers generated by the absorption of photons from illumination by the red portion of the solar spectrum, being more uniformly dispersed throughout the bulk of the photoactive region of the semiconductor material, are concentrated at a lesser density than are the charge carriers generated by the absorption of photons from illumination of blue portions of the solar spectrum. While the total number charge carriers generated in the semiconductor material of the photovoltaic cell under steady state conditions may be substantially equal if equal fluxes of red or blue photons enter and are absorbed by the semiconductor material of the cell, the peak rate of charge carrier recombination under red illumination is lower, since charge carriers generated by red illumination are more uniformly dispersed throughout the bulk of the semiconductor material, and are, therefore, less likely to encounter a charge carrier of opposite polarity at a recombination center than are charge carriers generated by blue illumination. In summary: the effective lifetime of charge carriers is dependent upon the wavelength of the illumination creating them. The lifetime is minimum where recombination is maximum, and this point of maximum recombination will depend upon the absorption profile of light in the photoactive region of the semiconductor material of a photovoltaic device.
Based upon the observations enumerated hereinabove, applicants conclude that it is necessary to promote the more uniform absorption of all photons of light from the solar spectrum throughout the bulk of the photoactive region of the semiconductor material, especially the absorption of photons of blue light. In this manner, more uniform generation of charge carriers throughout the bulk of the photoactive region of the semiconductor material is promoted. High charge carrier density in a narrow portion of the photoactive region is thus prevented, and the rate of charge carriers recombination at defect sites is decreased. The result is the fabrication of a photovoltaic cell exhibiting increased operational tolerance to defect states and hence improved stability (which translates into increased photogenerative efficiency).
It is well known that when a beam of light passes through a homogeneous absorbing medium, the intensity of that beam of light decreases exponentially with the absorption thereof throughout the medium, the greatest absorption occuring most proximate the light incident surface of the medium. According to the principles of the instant invention, there is provided a photovoltaic device exhibiting uniform absorption of light throughout the photoactive region of the semiconductor material thereof. As used herein, the term "uniform absorption of light" will refer to an absorption of light that deviates from the aforementioned exponential pattern. For example, the decrease in light intensity as said light travels through the absorbing medium may be linear, that is to say, at a hypothetical point half way through the bulk of the absorbing medium, a beam of light will have half the intensity of said beam of light at the light incident surface of the absorbing medium. Of course, the term "uniform absorption of light" is not meant to be solely limited to light absorption which follows such a linear relationship, but, rather, is meant to include all light absorption patterns that deviate from the normal exponential attenuation of light through a homogeneously absorbing medium. It is the essence of the instant invention to promote the uniform absorption by forming those regions of the absorbing medium most proximate the light incident surface thereof more transparent to incident radiation than those portions of the absorbing medium more distal from said light incident surface. Techniques for, and photovoltaic structures produced by, promoting the uniform absorption of light throughout the photoactive region of the semiconductor material of a photovoltaic cell will be described in greater detail hereinbelow.
More particularly, the instant invention provides for the uniform absorption of light throughout at least a substantial portion of the bulk of the photoactive region of the semiconductor material of a photovoltaic device by grading the band gap of the intrinsic layer of that device. A graded band gap intrinsic layer is one in which the band gap of the semiconductor material from which that layer is fabricated varies spatially; i.e., the band gap of the intrinsic semiconductor material taken in planes parallel to the plane of the intrinsic layer will vary relative to the thickness of that intrinsic layer. According to the principles set forth herein, the band gap of the intrinsic layer is graded so as to provide a relatively wide band gap region (for instance 1.9 eV) proximate the light incident surface of the photoactive region of the semiconductor material of the photovoltaic device and a narrower band gap region (for instance 1.7 eV) in the path of travel of the beam of incident light from the light incident surface. It should be noted that the term "graded band gap", as used herein, is intended to include any change in the band gap of the semiconductor material relative to the thickness thereof and specifically includes (1) a smooth variation in the band gap, be it a linear or nonlinear change, (2) a stepped variation in the band gap wherein said band gap varies in a series of two or more discrete steps, as well as (3) any combination of smooth and stepped band gap gradations. By grading the band gap of the intrinsic layer of semiconductor material of a p-i-n-type photovoltaic device, absorption of incident light, especially the more easily absorbed blue light, is spaced throughout at least a substantial portion of the bulk of the photoactive region thereof and the effective lifetime of charge carriers resulting from the absorption of photons from that illumination is increased. The result is the fabrication of an improved photovoltaic device exhibiting increased tolerance to defect states in the band gap thereof. It should be noted that it is not necessary to grade the band gap of the entire layer of intrinsic semiconductor material in order to promote said uniform photon absorption throughout substantially all of the bulk of the photoactive region. This is because the strongest photon absorption in amorphous silicon or amorphous silicon:germanium alloy layers occurs in a relatively narrow (i.e. 1000-2000 angstrom) portion of those layers; accordingly, by rendering this portion of the photoactive semiconductor layer relatively more transparent to incident radiation the uniform absorption of photons, as hereinabove defined, is promoted.
As mentioned supra, in the preferred embodiment band gap grading is accomplished by compositionally varying the intrinsic semiconductor material. For example, a band gap broadening element may be added to at least the portion of the photoactive intrinsic semiconductor material most proximate the light incident surface of the photovoltaic device to render those portions least absorbtive of incident radiation. Alternatively, the layer of intrinsic semiconductor material may be initially formed of a relatively wide band gap semiconductor material and a band gap narrowing element added to those portions of the photoactive region of that intrinsic layer most distal from the light incident surface of said photovoltaic device. By controlling the concentration of the band gap modifying element added to any given portion of the intrinsic layer, the width of the band gap thereof may be controlled. By smoothly varying the concentration of the band gap modifying element in the intrinsic semiconductor material, a smoothly graded band gap may be achieved. Similarly, by varying the concentration of the band gap modifying element in the previously described stepped manner, a stepped gradation of the band gap of the intrinsic semiconductor material may be achieved. Such techniques of band gap grading (also referred to as "profiling") will be described in greater detail hereinbelow.
A photovoltaic device having a varying band gap in a relatively narrow portion of the photoactive region of the layer of intrinsic semiconductor material is disclosed in a paper entitled, Achievement Of Higher Efficiency Amorphous Silicon-Germanium Solar Cells Using Affinity Gradients, presented by S. Wiedeman and E. A. Fagen at the 17th Annual I.E.E.E. Photovoltaic Specialists Conference held May 1-4, 1984 in Kissimmee, Fla. Disclosed therein is a n-i-p-type photovoltaic device formed of an amorphous silicon-germanium alloy in which the composition of the intrinsic semiconductor layer was profiled over the first few hundred angstroms from the light incident surface thereof. This band gap variation was accomplished by gradually altering the ratio of silicon to germanium in those few hundred angstroms. The object of such band gap variation is to establish an electrical field of varying strength adjacent the light incident surface of the intrinsic semiconductor material, which field is adapted to eliminate charge carrier losses at the interface of the n doped and intrinsic layer interface due to back diffusion of those charge carriers across the n and intrinsic interface. The authors of the paper believed that, because of the electrical field, a 29% improvement in the initial conversion efficiency of the photovoltaic devices was achieved. It should be noted that no claim was implicitedly or explicitely presented by the authors of the aforementioned paper for the improved long term stability of photovoltaic devices thus fabricated. This may be due to the fact that it is not possible to achieve improved long term stability from the reported structure since, as described hereinabove, losses in conversion efficiency are due to the bulk recombination of charge carriers at defect sites rather than a surface phenemonon such as back diffusion at semiconductor material layer interfaces. The method described by Wiedeman, et al has, as its object, the elimination of the back diffusion of charge carriers across the n doped and intrinsic layer interface, and, accordingly, compositional variation of the intrinsic semiconductor material is restricted to the immediate vicinity of that interface. By limiting the profile of the intrinsic semiconductor layers to, at best, the first few hundred angstroms, the majority of blue light would still be absorbed adjacent the light incident surface of that layer. Therefore, the method disclosed by Wiedeman, et al is not intended to and does not have any effect upon the recombination of charge carriers throughout the bulk of at least a portion of the intrinsic semiconductor material due to the presence of defect sites, such as those caused by photogradation.
As should be apparent from the foregoing discussion, there exists a definite need for an amorphous thin film photovoltaic device which (1) exhibits long term stability in the conversion efficiency thereof, (2) does not require annealing to achieve that stability, and (3) does not necessitate the inclusion of extraneous hardware to maintain that stability. The instant invention provides such an amorphous thin film photovoltaic device in which the absorption of light in at least a substantial portion of the photoactive region of the semiconductor material thereof is substantially uniform. The photovoltaic device of the instant invention is tolerant of photoinduced or photogenerated defects formed during operation and exhibits long term stability in its photoconversion efficiency.
These and many other advantages of the instant invention will be apparent from the drawings, the detailed description of the invention and the claims which follow.